NGC 6231 is a bright open cluster of approx. 120 stars spread over 15 arc minutes of sky in Scorpius. With a combined magnitude of +2.6 the group is visible to the naked eye and a stunning object through binoculars and telescopes. At its heart are numerous hot luminous fifth and sixth magnitude blue-white type B supergiants. They appear similar to a smaller version of M45, the famous Pleiades cluster in Taurus.

NGC 6231 is located half a degree north of bright star Zeta Scorpii whose Bayer designation is shared by zeta1 Scorpii (ζ1 Sco - mag. +4.7) and zeta2 Scorpii (ζ2 Sco - mag. +3.6). The two stars are separated by 7 arc minutes but this is no physical double star, just a line of sight effect. Zeta2 is much closer at 155 light-years while zeta1 - an outlying member of NGC 6231 - lies 5,700 light-years distant. NGC 6231 is also known as the Northern Jewel Box.

NGC 6231 - Open Cluster (credit - Australian Astronomical Observatory / David Malin)

Finder Chart for NGC 6231 (credit - freestarcharts)

Finder Chart for NGC 6231 - pdf format (credit - freestarcharts)

NGC 6231 was discovered by Giovanni Batista Hodierna sometime before 1654. Edmond Halley independently rediscovered in 1678. It was observed by Philippe Loys De Chéseaux in 1745-46 and Nicholas Louis de Lacaille in 1751-52. The cluster was just too far south to be seen by Charles Messier.

Through popular 7x50 or 10x50 binoculars it appears bright and condensed with many stars resolvable. With a small 80mm (3.1-inch) scope, NGC 6231 is superb with many more stars visible including a nice double towards the centre. Its resemblance to the Pleiades is apparent and if it were as close to us as the Pleiades, it would be the same size in the sky but 50x brighter. NGC 6231 is connected to a larger scattered cluster of fainter stars called H12, which lies 1 degree to the north. The chain of stars linking the two clusters outlines one of the spiral arms of our galaxy.

NGC 6231 is a spectacular open cluster 5,900 light-years distant. It has a spatial diameter of 26 light-years and is number 76 in the Caldwell catalogue. The cluster is best seen from southern and equatorial regions during the months of June, July and August.

NGC 6231 Data Table

NGC6231
Caldwell76
NameNorthern Jewel Box
Object TypeOpen Cluster
ConstellationScorpius
Distance (ly)5,900
Apparent Mag.2.6
RA (J2000)16h 54m 00s
DEC (J2000)-41d 48m 00s
Apparent Size (arc mins)15 x 15
Radius (light-years)13
Age (years)3.2 Billion
Number of Stars120
Other NameCollinder 315

Sky Highlights - March 2017

Comet
Comet 41P/Tuttle-Giacobini-Kresak now visible with binoculars as it heads towards perihelion

Mercury
Mercury heading towards greatest elongation east

Minor Planet
Vesta now visible with binoculars and small telescopes.

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for March 2017

Northern Hemisphere
Evening
West:- Venus (mag. -4.8 to -4.1 - first half of month), Mars (mag. +1.3 to +1.5), Uranus (mag. +5.9), Mercury (mag. -1.5 to -0.4 - second half of month)
Midnight
Southeast:- Jupiter (mag. -2.3 to -2.5)
Morning
Southwest:- Jupiter
Southeast:- Saturn (mag. +0.5)

Southern Hemisphere
Evening
West:- Venus (first half of month), Mars, Uranus
Midnight
North:- Jupiter
East:- Saturn
Morning
West:- Jupiter
Northeast:- Saturn
East:- Neptune (mag. +8.0 - second half of month)

Deep Sky
Naked eye / binoculars:-
Melotte 111 - Mel 111 - The Coma Star Cluster (Open Cluster)
Messier 44 - M44 - The Praesepe (Open Cluster)

Telescopes:-
Messier 67 - M67 - Open Cluster
Messier 51 - M51 - The Whirlpool Galaxy (Spiral Galaxy)
Messier 97 - M97 - The Owl Nebula (Planetary Nebula)
Messier 101 - M101 - The Pinwheel Galaxy (Spiral Galaxy)
Messier 65 – M65 – Spiral Galaxy
Messier 66 - M66 - Intermediate Spiral Galaxy
Messier 95 - M95 - Barred Spiral Galaxy
Messier 96 - M96 - Intermediate Spiral Galaxy
NGC 4244 - Spiral Galaxy
NGC 4565 - Needle Galaxy - Spiral Galaxy

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